Events with QGraphicsItemGroup - c++

Question

In my application I want to use QGraphicsItemGroup for grouping items into one item.
I played with it a little and not sure using it because when I want to catch events, events are merged together but I want to handle specific event with specific child.
How can I achieve this?

Answer 1

You need to call QGraphicsItemGroup::setHandlesChildEvents(false). This stops the QGraphicsItemGroup trying to handle the event, and lets the child QGraphicsItems handle them instead.

Answer 2

I think that's the point of the QGraphicsItemGroup. Judging from the documentation, this is meant to simplify moving and transforming multiple items at once e.g. imagine the following case: a user draws a selection rectangle around several items in an application because he wants to move all of them. Perhaps what you want more is to create a hierarchy of items, e.g. have one parent item with several child items. This way you'll get the individual events for each item. This can be accomplished by calling QGraphicsItem::setParentItem();

Answer 3

The question doesn't specify which version of Qt is concerned and there is a correct answer for Qt4. Here is an answer for Qt5 (it works for PyQt5 and I guess it'll work also in C++). The solution was to reimplement sceneEvent, reimplementing a specialized event-catcher such as contextMenuEvent was not sufficient.
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from PyQt5 import QtCore
from PyQt5 import QtGui
from PyQt5 import QtWidgets
class GraphicsItem(QtWidgets.QGraphicsItem):
def __init__(self,
rect: QtCore.QRectF,
name: str,
parent: QtWidgets.QGraphicsItem = None):
super().__init__(parent)
self._name = name
self._rect = rect
def boundingRect(self):
return self._rect
def paint(self,
painter: QtGui.QPainter,
option: QtWidgets.QStyleOptionGraphicsItem,
widget: QtWidgets.QWidget = None):
painter.setPen(QtGui.QPen(QtCore.Qt.NoPen))
painter.setBrush(QtGui.QBrush(QtCore.Qt.red))
painter.drawRect(self._rect)
def sceneEvent(self, event: QtCore.QEvent):
if (event.type() == QtCore.QEvent.GraphicsSceneContextMenu):
self.contextMenuEvent(event)
event.accept()
return True
def contextMenuEvent(self, event: QtWidgets.QGraphicsSceneContextMenuEvent):
print(f'contextMenuEvent in "{self._name}"')
class MainWindow(QtWidgets.QMainWindow):
def __init__(self):
super().__init__()
self._scene = QtWidgets.QGraphicsScene()
layout = QtWidgets.QHBoxLayout()
self._view = QtWidgets.QGraphicsView(self._scene)
layout.addWidget(self._view)
self._widget = QtWidgets.QWidget()
self._widget.setLayout(layout)
group = QtWidgets.QGraphicsItemGroup()
self._scene.addItem(group)
scene_item = GraphicsItem(QtCore.QRectF(0, 0, 100, 100), 'in scene')
self._scene.addItem(scene_item)
group_item = GraphicsItem(QtCore.QRectF(150, 0, 100, 100), 'in group')
group.addToGroup(group_item)
group_item = GraphicsItem(QtCore.QRectF(300, 0, 100, 100), '2nd in group')
group.addToGroup(group_item)
self.setCentralWidget(self._widget)
self.setWindowTitle('contextMenuEvent with QGraphicsItemGroup')
if __name__ == '__main__':
import sys
app = QtWidgets.QApplication(sys.argv)
mainWindow = MainWindow()
mainWindow.setGeometry(100, 100, 800, 500)
mainWindow.show()
sys.exit(app.exec_())